Question

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br} \frac{(\text { i }) \mathrm{A}_{\mathrm{B}} \mathrm{CN}}{\left(\text { ii } \mathrm{H}_{2} / \mathrm{Ni}\right.}{\longrightarrow}\) product. The product is (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}\left(\mathrm{NH}_{2}\right) \mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHCH}_{3}\)

Short answer

Short Answer: The final product of the sequence of reactions starting with CH3CH2Br is CH3CH2NH2.

Step-by-step solution

Reaction with A and B in the presence of CN-

The starting compound, CH3CH2Br, reacts with A and B in the presence of CN-. This reaction is a nucleophilic substitution, with the CN- ion acting as the nucleophile. The CN- ion will attack the electrophilic carbon (C2) attached to the bromine, and the C-Br bond will break, resulting in the formation of a new C-CN bond. The product of this reaction would be CH3CH2CN.

Reaction with H2 in the presence of a Ni catalyst

The intermediate product from the previous step, CH3CH2CN, now reacts with H2 gas in the presence of a nickel catalyst. This is a hydrogenation reaction, in which the carbon-nitrogen triple bond in the nitrile group will be reduced to a primary amine, -NH2 group. The product of this reaction would be CH3CH2NH2. The final product is CH3CH2NH2. Therefore, the correct option is: (a) CH3CH2NH2

Key concepts

Nucleophilic Substitution

Understanding nucleophilic substitution is essential in organic chemistry, as it's a fundamental reaction involving the exchange of one nucleophile (a 'lover of nuclei') for another on a carbon atom. Think of nucleophiles as species that are rich in electrons and seeking an electron-deficient atom to bond with, typically a carbon with a leaving group, like a halide. In the scenario provided, the nucleophile is the cyanide ion (CN-).

During the reaction, CN- targets the carbon atom bonded to bromine in ethyl bromide (CH3CH2Br). Bromine is highly electronegative, making the carbon it's attached to electropositive and an ideal target for nucleophilic attack. As CN- forms a bond with this carbon, bromine is pushed out. This displacement results in a new compound, ethyl cyanide (CH3CH2CN), through a reaction mechanism known as the SN2 pathway, which is common in primary alkyl halides like the one in our example.

Hydrogenation Reaction

A hydrogenation reaction involves the addition of hydrogen (H2) across a multiple bond. It's used to reduce or saturate organic compounds. In the hydrogenation of nitriles, the goal is to introduce hydrogen atoms to a molecule containing a carbon-nitrogen triple bond.

This process often requires a catalyst to proceed at a reasonable rate and temperature. The nickel (Ni) catalyst used in the example is a common choice for facilitating hydrogenation. It provides a surface for the reactants to adsorb onto, bringing the hydrogen in close proximity with the nitrile group, allowing the carbon-nitrogen triple bond to be broken and hydrogen to be added to the molecule. The result of such a hydrogenation is the transformation of the nitrile into an amine, specifically a primary amine when starting from a nitrile.

Nitrile Reduction

A fundamental transformation in organic chemistry is the reduction of nitriles. Reduction, in a broad sense, means adding electrons to a molecule, often accompanied by a gain of hydrogen or loss of oxygen. Here, we're focusing on adding hydrogen to a nitrile to convert it into a different functional group.

The nitrile group, which contains a carbon-nitrogen triple bond, is quite reactive due to its polarity and the high electronegativity of nitrogen. When a nitrile undergoes reduction, like with hydrogen gas in the presence of a Ni catalyst, the triple bond is broken and hydrogen atoms are added to both carbon and nitrogen, leading to a primary amine. This is a two-step process: first, the nitrile is reduced to an imine; then, the imine is further reduced to an amine. Given the profound impact this reaction has on the functionality of the molecule, it's widely used in the synthesis of amines from carbonitriles.

Primary Amine Synthesis

The synthesis of primary amines is a typical task in organic synthesis, and there are several methods to achieve this. The method demonstrated in the example uses a two-step process. First, a nitrile is formed through nucleophilic substitution, replacing a leaving group with the CN- ion on an alkyl chain. Subsequently, the nitrile group is reduced (as discussed in the 'Nitrile Reduction' section).

Primary amines have the formula RNH2, with R representing an alkyl or aromatic group. This type of synthesis is particularly valuable because amines are foundational to many biological molecules and pharmaceuticals. By carefully choosing the starting materials and reaction conditions, chemists can create a wide range of amines to use in further chemical synthesis or as final products in their own right.